Cardiac arrhythmias such as atrial fibrillation or focal atrial tachycardia affect a significant amount of the population. It is estimated that between 1 and 1.5% of the world's population are affected by atrial fibrillation. 25% of people aged 40 or older will develop atrial fibrillation. The heart condition is caused by disordered electrical activity in the atrial chambers of the heart. Symptoms of atrial fibrillation include shortness of breath, palpitations, dizziness, light-headedness, chest pain, chest discomfort, fatigue, anxiety, depression and increased side effects from drugs. Furthermore, it is estimated that atrial fibrillation causes 20-30% of all strokes and increases the risk of strokes by a factor of 5 compared to those without atrial fibrillation.
There are several known methods for treating atrial fibrillation. Some drugs have been found to reduce the symptoms, although the effectiveness of such drugs is variable and such drugs also exhibit side effects. A procedure known as DC cardioversion can trigger the heart into re-establishing normal rhythm for a relatively short amount of time. However, around 70% of patients treated with DC cardioversion revert to exhibiting atrial fibrillation within one year of the DC cardioversion. Lastly, ablating the walls of the atrial chambers has been found to be effective. Ablation is the only currently known therapy that can achieve long term freedom from atrial fibrillation.
The cardiac ablation technique usually involves delivering a transvenous catheter, often via the groin, to the heart and applying energy to the catheter tip so as to ablate the inner heart walls. In other approaches an ablation device can be applied by a surgeon to the external walls of the cardiac chambers. In some types of atrial fibrillation ablation the posterior wall of the left atrium is electrically isolated using an ablating catheter or device. The aim of the surgeon performing the ablation procedure is to draw a “fence” with the catheter so as to create a line of ablation that prevents electrical signals from crossing the fence. An island of the atrial wall can be cut-off electrically from the rest of the heart by drawing a circle, square or triangle (or other closed shape) with the catheter to isolate an expanse of tissue.
The technique is difficult and requires surgical skill. The ablation is usually done while the heart is beating and the heart walls are slippery and irregular. If the ablation is done with a catheter having an ablating tip in the form of a point, then many point ablations need to be made in a line. Each point ablation typically takes 15-20 seconds to be made. Although the surgeon usually has x-ray fluoroscopy and 3D mapping visualisation available during the procedure, it is nevertheless difficult for the surgeon to be sure that an unbroken line of ablations has been created on the heart surface.
Once the ablation procedure has been completed, it is useful to know whether it was successful. This is particularly the case 2-3 months after the ablation has taken place. In some circumstances, some areas of heart tissue may “recover” so that the posterior wall of the left atrium is no longer electrically isolated, even though it was deemed isolated at the time of the ablation surgery.
The procedure for determining whether an ablation procedure was successful is not simple. Typically, a mapping catheter (usually having an electrode arranged in a spiral configuration) is passed up the femoral vein from the groin into the left atrium via an atrial transseptal puncture hole created in the heart by a steel needle. A pacing catheter is also inserted via the same route. Both the pacing catheter and the mapping catheter are then placed so as to contact the atrial wall at a location that is “inside” the area that the fence is supposed to surround. The pacing rate is selected to be above the natural rhythm of the heart. While inside the area of the fence, it would be expected that the mapping catheter is able to receive the electrical signals transmitted by the pacing catheter through the heart walls. It would also be expected that the heart does not adopt the pacing rate set, as the area being paced is supposed to be electrically isolated. The mapping catheter can then be moved to a location outside of the area surrounded by the fence and it can be determined whether electrical signals are able to cross the fence or not. If they cannot cross the fence (and the heart does not adopt the applied pacing), electrical isolation can be confirmed.
This procedure can be done either while under anaesthesia or when conscious. It represents a lengthy and highly invasive procedure with a potential risk of serious complications. Furthermore, it is an expensive procedure.
It would thus be desirable if there existed a way to determine the ablation condition that did not rely on expensive, invasive and risky surgical procedures.